Historical patterns of oak population expansion in the Chautauqua Hills, Kansas

ORIGINALARTICLE

Historical patterns of oak populationexpansion in the Chautauqua Hills,KansasThomas R. Rogers and F. Leland Russell*

Department of Biological Sciences, Wichita

State University, Wichita, KS 67260, USA

*Correspondence: F. Leland Russell,

Department of Biological Sciences, Wichita

State University, Wichita, KS 67260, USA.

E-mail: [email protected]

ABSTRACT

Aim Rates of tree population expansion have increased in many North Ameri-

can landscapes that were mosaics of grasslands, savannas and woodlands his-

torically. Consequences of woodland expansion include reduced economic

return from grazing and changes in native biodiversity, but causes of woodland

expansion are poorly understood. We address historical timing of blackjack

oak (Quercus marilandica) and post oak (Quercus stellata) population expan-

sion in tree–grass ecosystems, roles of climate and land use in driving this

expansion, and future stability of these woodlands.

Location The Cross Timbers ecosystem in Kansas, USA.

Methods Using increment cores, we quantified blackjack oak and post oak

age structures in four woodlands on sites that were not wooded in the 1860s.

We compared timing of oak regeneration with climate fluctuations (using the

Palmer drought severity index) and land-use history. We quantified tree species

composition within 5-m radii of sampled oaks.

Results Recruitment of both oak species increased between 1925 and 1945.

Modal age classes recruited from 1935 to 1960. Recruitment was associated

with dry intervals at the two sites with north- or east-facing aspects. This asso-

ciation was driven by blackjack oak recruitment in dry intervals. Woodlands

on the sites with south- and west-facing aspects contained only oaks, whereas

those on the sites with north- and east-facing aspects contained saplings of

fire-intolerant, shade-tolerant tree species.

Main conclusions Our results contribute to growing evidence for woodland

expansion in the region during dry climate intervals. The association between

drought and recruitment was influenced by slope aspect and was more pro-

nounced in the less fire-tolerant oak species. Although woodland expansion

coincided with regional increases in fire frequency, drought and greater use of

prescribed burning are likely to have reduced fire intensities by reducing fuel

loads. These oak woodlands, which have developed during the 20th century,

appear to form stable communities on xeric slopes but to be undergoing suc-

cession towards a mesophytic tree community on mesic slopes.

Keywords

Age structure, Cross Timbers, fire ecology, Kansas, mesophication, Quercus

marilandica, Quercus stellata, tree recruitment, woodland dynamics, woody

plant encroachment.

INTRODUCTION

Landscapes that are mosaics of grasslands, savannas and

woodlands typically are dynamic, with the physiognomy of

individual patches changing on time-scales from decades to

centuries (Callaway & Davis, 1993; Moustakas et al., 2009).

Such shifts in physiognomy can be associated with substan-

tial changes in ecosystem functioning, such as changes in

ª 2014 John Wiley & Sons Ltd http://wileyonlinelibrary.com/journal/jbi 2105doi:10.1111/jbi.12360

Journal of Biogeography (J. Biogeogr.) (2014) 41, 2105–2114

hydrological and biogeochemical processes (Archer et al.,

2001; Huxman et al., 2005), and in ecosystem structure, such

as levels of native biodiversity (Ratajczak et al., 2012). While

transitions among all three physiognomies occur in land-

scape mosaics, rates of woody plant establishment in grass-

lands and savannas have increased substantially within the

last 50–300 years, with examples from all continents except

Antarctica (Archer et al., 1988). Globally, humans use tree–

grass ecosystems extensively for raising livestock, and there-

fore shifts from grassland or savanna to woodland are of

economic interest because of the reduction in forage (Bok-

dam et al., 2000; Dube & Pickup, 2001). Although causes of

woody plant expansion remain difficult to identify, contrib-

uting factors include changes in human land use (McPherson,

1997; Hessl & Graumlich, 2002; Abrams, 2003) and climate

changes (Archer et al., 2001) that alter tree–grass

interactions.

Fire and grazing influence strengths and outcomes of

tree–grass competitive interactions and, hence, can deter-

mine rates of woody plant encroachment. High-intensity,

frequent fires favour herbaceous vegetation and exclude

woody plants because most juvenile woody plants are sus-

ceptible to fire (Knapp et al., 2009). Rangeland management

may involve prescribed fire where fire suppression and habi-

tat fragmentation have eliminated wildfires, but managed

burns are often conducted when fuel loads are low, produc-

ing fires that are not sufficiently intense to suppress woody

plant recruitment (Knapp et al., 2009). Grazing can have

complex direct and indirect effects on woody plant–grass

interactions. Some of these effects may inhibit woody plant

establishment, such as the consumption of seedlings and

increased evaporation from the soil surface, and others may

promote woody plant establishment, such as reduced herba-

ceous competition and less fuel for fires (Riginos & Young,

2007; Cipriotti & Aguiar, 2012). Where cattle are the princi-

pal grazers, increased grazing is hypothesized to promote

woody plant establishment because cattle do little browsing

(Riginos & Young, 2007).

Climate fluctuations can play a critical role in woodland

expansion into grasslands and savannas. High precipitation

and low evapotranspiration can alleviate grass–woody plant

belowground competition and reduce fire frequencies and

intensities by keeping fuels moist. These mechanisms under-

lie a long-standing hypothesis that woody plant recruitment

will be greater during wet, cool climates (Archer et al., 1988;

McPherson & Wright, 1990). Recent results, however, suggest

an association between drought and woody plant recruitment

in pyrogenic grassland–savanna–woodland landscapes (Zie-

gler et al., 2008; Shuman et al., 2009). Low soil moisture

availability may reduce herbaceous biomass production,

reducing fire intensities and allowing woody plants to survive

fires (Ziegler et al., 2008). The ecological conditions under

which moisture and drought are each associated with woody

plant population expansion remain poorly understood.

In relatively mesic grassland–savanna–woodland land-

scapes, sustained changes in fire regimes may not only allow

woodland expansion, but may also promote woodland suc-

cession to forests dominated by fire-intolerant, shade-tolerant

tree species. Nowacki & Abrams (2008) refer to this shift

among alternate stable states in woodlands as ‘mesophica-

tion’. Throughout much of the eastern deciduous forest

region, oak forests are undergoing succession as fire-intolerant,

shade-tolerant tree species come to dominate the understo-

rey and densities of oak saplings and small adults are

reduced. Nowacki & Abrams (2008) hypothesized that oak

woodlands may be a stable community in eastern North

America only on xeric sites. Nevertheless, recent research in

the Oklahoma Cross Timbers region suggests replacement of

oaks by eastern red cedar (Juniperus virginiana) and calls

into question the stability of oak woodlands at the western,

xeric limit of eastern deciduous forest in North America

(DeSantis et al., 2011).

The Cross Timbers ecosystem in Texas, Oklahoma and the

Chautauqua Hills of south-eastern Kansas, USA, represents

the western and southern portion of the boundary between

eastern deciduous forest and prairie in North America. This

ecotone historically was a mosaic of grassland, savanna and

woodland (Dyksterhuis, 1949). Blackjack oak (Quercus mari-

landica Muenchh.) and post oak (Quercus stellata Wang.) are

the dominant tree species in the Cross Timbers. These spe-

cies belong to separate oak subgenera that differ in drought-

and fire-tolerance (Johnson et al., 2009). In the Chautauqua

Hills comparisons of modern plant communities with vegeta-

tion data from General Land Office (GLO) records from the

1860s suggest that tree cover has increased. The ecosystems

of the Chautauqua Hills have experienced profound land-use

changes and large climate fluctuations over the past

150 years that may have driven regional change in vegetation

physiognomy.

Here we quantify age structures of blackjack oak and post

oak populations to examine the historical timing and causes

of oak expansion in the Chautauqua Hills, Kansas, USA, and

to assess future successional change in these woodlands. We

address the following questions. (1) When did oak expansion

occur? (2) Do blackjack oak and post oak exhibit differential

timing of expansion? (3) Did oak expansion coincide with

climate fluctuations (drought versus high water availability)

or changes in land-use and fire regime? (4) Are these oak

woodlands stable or are they undergoing succession towards

a mesophytic tree community?

MATERIALS AND METHODS

Study species

Blackjack oak belongs to the subgenus Erythrobalanus (red

oak). Blackjack oaks are deciduous trees that grow 6–9 m

tall. Their range extends from New Jersey west to Iowa, east-

ern Kansas and central Texas and south to north-western

Florida (Harlow et al., 1996). In Oklahoma woodlands,

blackjack oaks of up to 120 years old occur (DeSantis et al.,

2011).

Journal of Biogeography 41, 2105–2114ª 2014 John Wiley & Sons Ltd

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T. R. Rogers and F. L. Russell

Post oak belongs to the subgenus Leucobalanus (white

oak). Post oaks are deciduous trees that reach 15–21 m in

height. This species occurs from southern New England west

to eastern Kansas and central Texas and south to central

Florida. Post oak is relatively drought-tolerant, but less so

than blackjack oak. In a literature review of factors affecting

oak regeneration in eastern North America, McQuilken

(1983) ranked blackjack oak as most drought-tolerant and

post oak as second-most drought-tolerant among seven

upland oak species in central Pennsylvania. Post oaks, as is

true for most species in the white oak subgenus, form tyloses

in fire-damaged vascular tissue, making them more fire-tolerant

than most species in the red oak subgenus (Johnson et al.,

2009). Post oak lifespans in the Cross Timbers can exceed

300 years (Therrell & Stahle, 1998).

Study sites

Criteria used to select study sites were (1) presence of both

blackjack oak and post oak, and (2) evidence of increased

tree cover in comparison with GLO records. Four sites were

selected: Cross Timbers State Park (37°440 N, 95°560 W;

hereafter ‘Cross Timbers SP’); Fall River State Lake

(37°390 N, 96°020 W; hereafter ‘Fall River’); Woodson State

Fishing Lake (37°470 N, 95°500 W; hereafter ‘Woodson’); and

Stotts’ Ranch (37°300 N, 96°010 W; hereafter ‘Stotts’). All

sites are on sandstone-derived soils. Fall River and Stotts are

in the westernmost counties in which blackjack oak and post

oak are recorded in Kansas (Great Plains Flora Association,

1977). Cross Timbers SP and Woodson are one county east

of these western range limits.

Cross Timbers SP and Woodson have been managed by

the Kansas Department of Parks and Wildlife since 1960 and

1937, respectively. Fall River has been managed by the Army

Corps of Engineers since 1949. At all three sites, prescribed

burning is conducted in a 3-year rotation with spot burning

to control invasion of woody plants and non-native species.

Stotts is privately owned. Between the 1930s and 1970s the

ranch was grazed heavily (Caleb Stotts, pers. comm.). Since

2000, stocking rates have been at or below recommended

National Resources Conservation Services levels. The current

prescribed burn strategy (2007–2011) involves burning

between two and four out of five years (Caleb Stotts, pers.

comm.), but historically burn strategies have varied. Guyette

et al. (2011) determined fire history for this site using tree

rings.

In GLO records, no timber is reported in the quarter sec-

tions where we sampled at Cross Timbers SP, Fall River and

Stotts. The vegetation mosaic at Woodson was more com-

plex. Surveyor notes for the sections that included our sam-

ple site at Woodson indicate that between 25% and 70% of

the lengths of the section boundaries were woodland, with

the remainder described as prairie. GLO surveys relevant to

Cross Timbers SP and Woodson were conducted in 1859

and surveys relevant to Fall River and Stotts were conducted

in 1867.

Field methods

Within each site, discrete woodland patches were defined by

natural or human-made boundaries (e.g. drainages, roads,

fences). Patch accessibility and size were considered in select-

ing one woodland patch to sample per site. At all four sites,

selected woodlands were on uplands that included first- and

second-order intermittent stream drainages, but not peren-

nial streams. Stands sampled at Cross Timbers SP and Fall

River occupied north- and east-facing aspects, while stands

at Stotts and Woodson occupied south- and west-facing

aspects. Using Google Earth, we determined the distance

between the northern- and southern-, and eastern- and wes-

tern-most points of each selected woodland. Maximum dis-

tances along north–south, east–west axes were used to

establish a coordinate system. We randomly selected a point

on this coordinate system to begin sampling trees. From this

point, we ran 100-m transects in the four cardinal directions.

The point-quarter method of sampling was used at 20-m

intervals along each of the four transects. At each 20-m

increment, the nearest blackjack oak and post oak tree was

sampled within each of four quadrants (80 trees species�1

site�1). If a transect exited the woodland before reaching

100 m, sampling on the next transect was extended to com-

pensate. Upon encountering a cluster of stems, possibly the

same genet, one stem was randomly selected for sampling.

Stems were considered to be the same genet if one stem

sprouted from the base of another, they were ≤ 30 cm apart,

or they were arranged in a circle and leaned away from each

other.

For each sampled tree, we measured its basal diameter, its

diameter at breast height (d.b.h.) and extracted an increment

core from the base of trees ≥ 4 cm d.b.h. If the increment

core collected from a tree did not pass through the centre of

the trunk then that tree was excluded from the age structure

and all analyses involving age. Therefore, actual sample sizes

were less than the 80 trees species�1 site�1 that our sampling

design should have provided. The minimum sample size

upon which age structures and analyses involving tree ages

were based was 41 trees, which occurred for blackjack oak at

Fall River and also at Cross Timbers SP.

To gain insight into spatial patterns of woody plant

expansion in uplands, we were interested in the relationship

between tree age and landscape features such as slope steep-

ness and topographic position. Therefore, for each tree sam-

pled we recorded slope position (ridge, mid-slope or

drainage) and measured slope steepness at the tree’s base

using a clinometer. To quantify tree species composition at

the sampling sites, the species and d.b.h. of all trees within

5 m of each sampled oak in the north-east quadrant at each

sampling point were recorded.

The samples that we collected to quantify age structures of

the two oak species included few trees that occurred in

drainages. Therefore, to evaluate the relationship between

topographic position and tree age, we augmented the sample

of trees in drainages at each site. We extended a pre-existing


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Oak population expansion in south-eastern Kansas

transect towards the nearest drainage where we randomly

determined which direction (upstream or downstream) and

which side of the drainage would be sampled. A 100-m tran-

sect was placed 10 m from the edge of the stream bed, and

at 20-m intervals along the transect sample trees were ran-

domly selected by drawing a piece of paper (labelled NE, SE,

NW or SW) corresponding to a quadrant in which the near-

est tree of each species would be cored. A total of five cores

from drainages were collected for each species at a site.

Across all four sites, 40 additional cores were sampled using

this method and were included in the landscape position

analysis.

Sampling occurred from June 2010 to July 2011. Incre-

ment cores were allowed to dry for ≥ 96 h, mounted, and

sanded with progressively finer (160–400 grit) paper. Ring

widths were measured using Coo Recorder (Cybis Elek-

tronic & Data AB, Saltsjobaden, Sweden) and were analysed

by visual cross-dating. Visual cross-dating of cores was con-

firmed using CDendro (Cybis Elektronic & Data AB) and

cofecha (Holmes, 1994). These programs are used to elimi-

nate age-related variation in radial growth rates, and to

detect missing or false rings.

Statistical analysis

We compared population age structures between the two

oak species at each site by using a Kolmogorov–Smirnov

two-sample test. To specifically compare mean age and mean

d.b.h. between the species at each site we used one-way

ANOVAs. Throughout our analyses, when multiple compari-

sons were made, as for separate Kolmogorov–Smirnov two-

sample tests and ANOVAs for each site, we used Bonferroni

adjustments to significance levels. Statistical analyses were

conducted using SAS Enterprise Guide 4.2.

To determine the historical landscape position of oaks, we

examined relationships between (1) tree age and slope steep-

ness, and (2) tree age and topographic position. We pooled

the oak species for these analyses. The relationship between

slope steepness and tree age was tested with an ANCOVA

using steepness as a covariate and site as a random effect.

For each site, we compared oak age among topographic posi-

tions (ridge, mid-slope and drainage) using ANOVA. Age

was square-root transformed for ANCOVAs and ANOVAs to

improve normality of the residuals and homoscedasticity.

To evaluate the relationship between water availability and

oak recruitment, we compared the proportion of trees at

each site that recruited in wet versus dry 7-year climate

intervals with the proportion of such intervals in the 1898–

2008 (the most recent year when a tree that we cored

recruited) climate record that were wet versus dry, as in Zie-

gler et al. (2008). We obtained Palmer drought severity index

(PDSI) values for Kansas climate region 9, which includes

our four study sites (NOAA, 2013). The year in which a tree

recruited was designated as part of a ‘wet’ climate interval if

the 7-year average PDSI, calculated using PDSI values for

3 years preceding the year of recruitment, the year of recruit-

ment and 3 years following the year of recruitment, was > 0.

If the 7-year average PDSI was < 0 then the year of recruit-

ment was designated as occurring in a ‘dry’ climate interval.

We used a 7-year running average of PDSI values in our

analyses because Ziegler et al. (2008) found PDSI 7-year run-

ning averages to predict timing of burr oak (Quercus macro-

carpa) regeneration at the eastern deciduous forest–prairie

ecotone in Minnesota, USA. First, for each site we used a

likelihood ratio test to determine whether blackjack oak and

post oak differed in the proportion of trees that recruited in

wet versus dry intervals. We then combined the species and

used likelihood ratio tests to compare the proportion of trees

that recruited in wet versus dry intervals with the proportion

of intervals that were wet versus dry for each site.

We used tree species composition within 5-m radii of oaks

that we sampled in the north-eastern quadrant at each sam-

pling point to quantify representation of mesophytic tree

species in the woodland. At sites where these 5-m radius cir-

cles included mesophytic species, we compared d.b.h. among

tree species using one-way ANOVA followed by Tukey tests.

RESULTS

Size structures

Diameter at breast height ranged from 3.2 to 52.3 cm for

both species across all four sites. The modal d.b.h. class for

both oak species was 15–20 cm at Cross Timbers SP, Fall

River and Stotts. At Woodson, the modal d.b.h. class was

10–15 cm for blackjack oak and 25–30 cm for post oak. The

oak species only differed in d.b.h. at Woodson, where post

oak was larger (F1,120 = 17.20; P < 0.001).

Age structures

Age distributions for the two oak species differed signifi-

cantly at Woodson (D = 0.498, P < 0.001) only. Mean age

of individuals sampled for the two species differed signifi-

cantly at two of the four sites, but the identity of the species

with the greater mean age differed among these two sites

(Table 1). Post oak was older at Woodson (F1,106 = 12.24;

P < 0.001) and blackjack oak was older at Stotts

(F1,107 = 6.47; P = 0.012). Mean age of post oak was margin-

ally significantly greater than blackjack oak at Cross Timbers

SP (F1,88 = 5.63; P = 0.020).

Post oak age structures were unimodal at Cross Timbers

SP, Fall River and Stotts (Fig. 1) with modal age classes of

65–70 years, 50–55 years and 25–30 years, respectively. The

post oak age structure at Woodson was bimodal with the

modal age class being 60–65 years and the second most fre-

quent age class being 145–150 years. Only the post oak pop-

ulation at Woodson included trees that were present before

GLO surveys. Four trees recruited from 1819 to 1850. Age

structures of blackjack oak were unimodal at all four sites

(Fig. 1), with modal age classes between 40 and 60 years at

all sites (Table 1).


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Landscape position effects

There was a marginally significant, negative relationship

between tree age and slope steepness (F1,3 = 9.88, P = 0.052).

However, this relationship was driven by three trees (out of

437 trees in the analysis) that were young and occurred on

the steepest slopes. Mean tree age did not differ between

drainages, mid-slopes and ridges at any site (P > 0.130).

Association with climate fluctuations

At Cross Timbers SP, blackjack oak recruitment was signifi-

cantly more associated with dry 7-year climate intervals than

was post oak recruitment [2lnL (likelihood ratio test statis-

tic) = 6.796, P = 0.009; Fig. 2]. The proportions of blackjack

oak and post oak recruiting during dry intervals at Cross

Timbers SP was 0.66 (SE 0.074) and 0.46 (SE 0.066), respec-

tively. Higher proportions of blackjack oaks than of post

oaks recruited during dry intervals at Fall River [blackjack:

0.61 (SE 0.076); post: 0.52 (SE 0.062)], Stotts [blackjack: 0.49

Table 1 Summary of descriptive statistics for blackjack oak

(Quercus marilandica) and post oak (Q. stellata) age structuresat the four study sites in the Chautauqua Hills, Kansas. At each

site, trees were sampled using the point-quarter method alongfour 100-m transects. The minimum diameter at breast height

(d.b.h.) for sampling was 4 cm.

Site Species

Age

range

(years)

Mean

(� SE)

age (years)

Modal

age class

(years)

% of

trees in

modal

age class

Cross

Timbers

Blackjack oak 17–86 57.0 (� 2.2) 50–55 26.8

Post oak 26–138 66.7 (� 2.9) 65–70 25

Fall River Blackjack oak 11–115 55.6 (� 2.9) 55–60 19

Post oak 5–148 57.9 (� 3.2) 50–55 17.6

Stotts Blackjack oak 11–83 41.1 (� 2.3) 40–45 15.7

Post oak 4–68 33.7 (� 1.6) 25–30 21.4

Woodson Blackjack oak 4–112 41.4 (� 3.2) 45–50 24

Post oak 6–193 80.5 (� 6.7) 60–65 13.6

Figure 1 Post oak (Quercus stellata) andblackjack oak (Q. marilandica) age

structures at the four Cross Timberswoodlands, Kansas. Five-year age categories

are used. At each site, trees were sampledusing the point-quarter method along four

100-m transects. The minimum diameter atbreast height (d.b.h.) for sampling was

4 cm.


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(SE 0.070); post: 0.40 (SE 0.058)] and Woodson [blackjack:

0.57 (SE 0.071); post: 0.50 (SE 0.077)], but these differences

in recruitment between the oak species were not statistically

significant.

After pooling the two species, oak recruitment was signifi-

cantly associated with dry intervals at Fall River (2lnL = 7.144,

P = 0.007; Fig. 2). The proportion of trees of both oak species

recruiting during dry climate intervals at Fall River was 0.56

(SE 0.048) and the frequency of years that were in the middle

of dry 7-year intervals in the 111-year regional PDSI record

was 0.42 (SE 0.047). After Bonferroni correction for separate

tests at the different sites, there was a marginally significant

association of oak recruitment with dry intervals at Cross Tim-

bers SP (2lnL = 5.096, P = 0.020; Fig. 2) where the proportion

of oak trees recruiting during dry intervals was 0.54 (SE 0.05).

This marginally significant association was driven by a very

strong association of blackjack oak recruitment and drought

(2lnL = 8.880, P = 0.003), whereas post oak recruitment was

associated with neither dry nor wet conditions (P = 0.661). At

Stotts and Woodson, oak recruitment was not associated with

dry or wet intervals. The proportion of oak trees recruiting

during dry intervals was 0.44 (SE 0.044) and 0.54 (SE 0.052) at

Stotts and Woodson, respectively.

Tree species composition

The two woodlands on mesic slopes, Fall River and Cross Tim-

bers SP, had greater tree species richness than the woodlands

on xeric slopes, Woodson and Stotts (Table 2). Only oaks were

encountered at Woodson and Stotts. Stem density of post oak

Figure 2 Climate fluctuation andrecruitment of blackjack oak (Q.

marilandica) and post oak (Q. stellata) atCross Timbers State Park and Fall River,

Kansas. The top panel presents 7-year(3 years before and after the year of tree

recruitment) running averages for Palmerdrought severity index values. At each site,

trees were sampled using the point-quartermethod along four 100-m transects. The

minimum diameter at breast height (d.b.h.)for sampling was 4 cm.


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was greater than all other tree species at every site except for

Stotts, where blackjack oak had the greatest density. At Cross

Timbers SP black cherry (Prunus serotina), eastern red cedar

and eastern redbud (Cercis canadensis) had higher stem densi-

ties than blackjack oak. At Fall River, black cherry, eastern red

cedar and eastern redbud were present, but at lower stem den-

sities than post oak and blackjack oak. At Cross Timbers SP,

mean d.b.h. values for post oak and blackjack oak were signifi-

cantly greater than the mean d.b.h. of any other tree species

(P < 0.001) except American elm (Ulmus americana). At Fall

River, mean d.b.h. values for post oak and blackjack oak were

significantly greater than for any other species (P < 0.001).

DISCUSSION

Temporal patterns of oak regeneration

Our results demonstrate prolific oak recruitment in the Chau-

tauqua Hills in the early- to mid-20th century. We suggest that

this episode of oak recruitment at our study sites represents

initial expansion of woodlands into patches of the landscape

mosaic that were savanna or grassland. It seems unlikely that

the recruitment wave that occurred in the early to mid-20th

century was stimulated by harvesting woodlands that grew

after GLO surveys in the 1860s. In agricultural schedules from

the 1925 Kansas state census no respondents reported income

in the previous year from marketing wood in the townships

that include Stotts (Painterhood township) and Woodson

(Belmont township). Only 1 of 138 respondents and 2 of 143

respondents marketed wood in the townships that include Fall

River (Salt Springs township) and Cross Timbers SP (Toronto

township), respectively. For these four townships, Kansas cen-

sus agricultural schedules show peaks in the proportion of

respondents that marketed wood in 1885 (6.9%, Painterhood),

1895 and 1905 (8.8%, Salt Springs), 1905 (8.8%, Toronto) and

1915 (15.4%, Belmont). Harvesting in the late 19th and early

20th centuries probably did not involve woodlands that

expanded into grasslands and savannas in the interval since

GLO surveys were conducted because such trees would have

been young and presumably small. Young woodlands at our

study sites could have been cut without marketing the wood,

but trends in marketing wood probably reflect general land use

practices with regard to woodlands.

In the oak woodlands at our study sites, age distributions

of blackjack oak populations and post oak populations were

generally similar, but subtle differences in age structures

appear to correlate with differences between the species’ phy-

siologies and life histories. Species in the red (Erythrobal-

anus) and white (Leucobalanus) oak subgenera differ in

drought tolerance, fire tolerance and lifespan (Johnson et al.,

2009). For blackjack oak and post oak, mean age of trees

differed at two sites, and marginally so at a third. However,

because the identity of the species with the greater mean age

differs between the sites, differences in mean age are unlikely

to reflect a successional sequence among the oak species, but

instead reflect the longer lifespan of post oak. Only at the

site with the youngest oak populations (Stotts) did blackjack

oak have a greater mean age than post oak, whereas at the

sites with the oldest oak populations (Cross Timbers SP and

Woodson) mean age of post oak was greater. We suggest

that populations of either species can predominate in the

early stages of woodland expansion or their spread can occur

Table 2 Description of tree community composition at the four study sites in the Chautauqua Hills, Kansas. Species composition data

were collected within circles of 5-m radii around oaks sampled for age structures and do not include the oak at the centre of the circle.All stems taller than breast height were recorded. ‘Relative stand proportions’ refers to the proportion of the total density of trees

contributed by each species. If no standard error is shown after mean diameter at breast height (d.b.h.) then only one individual of thespecies was sampled.

Site

Species

richness Species composition

Mean (� SE) d.b.h.

(cm)

Tree density

(stems/ha)

Relative stand

proportion

Cross Timbers 6 Post oak (Quercus stellata) 14.11 � 0.66 133.96 0.33

Blackjack oak (Q. marilandica) 15.89 � 2.21 28.72 0.07

Eastern redubud (Cercis canadensis) 3.52 � 0.45 36.36 0.09

Eastern red cedar (Juniperus virginiana) 4.43 � 0.66 70.8 0.17

Black cherry (Prunus serotina) 3.86 � 0.64 120.56 0.30

American elm (Ulmus americana) 11.13 � 1.65 9.56 0.02

Fall River 7 Post oak 17.27 � 0.82 280.84 0.70

Blackjack oak 23.13 � 2.44 51.08 0.13

Eastern redbud 4.97 � 1.53 29.80 0.07

Sugarberry (Celtis laevigata) 3.50 4.24 0.01

Eastern red cedar 3.55 � 0.38 34.04 0.09

Osage orange (Maclura pomifera) 4.46 4.24 0.01

Black cherry 8.03 � 2.05 21.28 0.05

Stotts 2 Post oak 18.04 � 0.97 197.48 0.49

Blackjack oak 20.28 � 0.94 202.52 0.51

Woodson 3 Post oak 16.58 � 1.17 262.08 0.66

Blackjack oak 10.67 � 0.94 101.16 0.25

Red oak (Quercus rubra) 19.51 � 2.84 36.80 0.09


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simultaneously, but as woodlands age blackjack oaks die and

older age classes become dominated by post oak.

Our results also provide evidence of differences, perhaps

subtle, in regeneration niche between these two dominant

oak species of Cross Timbers woodlands. At Cross Timbers

SP blackjack oak recruitment was significantly more associ-

ated with dry climate intervals than was post oak recruit-

ment. Further, although differences at no other site reached

statistical significance, at all sites the proportion of blackjack

oak that recruited during dry intervals was greater than for

post oak. This difference in regeneration timing is consistent

with greater drought tolerance in blackjack oak and, poten-

tially, with less fire tolerance. Ziegler et al. (2008) and Shu-

man et al. (2009) hypothesized that woody plant population

expansion would be greatest during dry periods because

sparse herbaceous biomass would support less-intense fires.

Species in the red oak subgenus, in general, are less fire-tol-

erant than species in the white oak subgenus. Ultimately, dif-

ferences between the species in the degree of association

between dry conditions and recruitment may have had subtle

effects on age structures because between 1910 and 1960 dry

and wet periods of 10–15 years duration alternated regularly

in south-eastern Kansas (NOAA, 2013).

Correlates and possible causes of oak population

expansion

The relative synchrony of increased recruitment across our

study sites indicates that oak population expansion was initi-

ated by regional processes. We suggest that oak expansion in

the Chautauqua Hills is best explained by the combination

of drought, changes in natural fire regimes in the late 19th

century, and land management changes during the economic

and environmental crises of the 1930s.

Climate fluctuations are thought to be important in shifting

patch physiognomy in grassland–savanna–woodland landscape

mosaics (Archer et al., 1988; Ziegler et al., 2008). Although

woody plant encroachment into grasslands and savannas has

been hypothesized to coincide with moist climate in tree–grass

ecosystems, Ziegler et al. (2008) found that drought promoted

woodland expansion at the forest–prairie ecotone in Minne-

sota, USA. Our results also suggest an association between

drought and oak recruitment at our sites with mesic slopes,

Cross Timbers SP and Fall River. On mesic sites, we would

expect herbaceous biomass production to be greater than on

xeric sites and, hence, drought might be necessary to reduce

standing crop herbaceous biomass and fire intensity below a

threshold at which juvenile oaks can survive fire.

DeSantis et al. (2011) hypothesized that, historically,

drought allowed regeneration in oak woodlands of the Cross

Timbers and further west in Oklahoma. In existing woodlands,

severe droughts may kill adult oaks, increasing light penetra-

tion. Once drought abates, renewed moisture and high light

conditions in the understorey may provide an opportunity for

oak recruitment. Because GLO surveys for Cross Timbers SP

and Fall River do not indicate woodlands in the quarter sec-

tions that include our sites, we think that the mechanism pro-

posed by Ziegler et al. (2008) is more relevant for our sites.

Our results complement those of DeSantis et al. (2011) by

addressing the role of drought in oak regeneration, not in

existing woodlands, but in components of the historical land-

scape mosaic that were grassland or savanna.

Reduced herbaceous fuel loads associated with dry condi-

tions in the early to mid-20th century would probably have

reinforced reductions in fire intensities as prescribed fire was

increasingly used in land management. In contrast to less fre-

quent fires throughout much of eastern North America dur-

ing the 20th century (Nowacki & Abrams, 2008), fire

histories for Cross Timbers sites (Stambaugh et al., 2009;

DeSantis et al., 2010; Allen & Palmer, 2011; Guyette et al.,

2011) indicate that fires were more frequent in the 20th cen-

tury than in recent preceding centuries. Further, Guyette

et al. (2011) found that fire scars at Stotts occurred at more

frequent intervals than at any comparable site that they sam-

pled in the Great Plains. However, fires were probably less

intense in the Cross Timbers during the 20th century than

preceding centuries as a result of changes in the season of

fire (Allen & Palmer, 2011). Knapp et al. (2009) suggest that

before Euro-American settlement of the Great Plains, fires

occurred during both the growing and dormant seasons.

During the 20th century and today, prescribed burns are typ-

ically conducted in late winter or early spring in the Chau-

tauqua Hills (Guyette et al., 2011). Annual early spring fires

did not kill established shrubs, but instead allowed their slow

expansion in cover (Heisler et al., 2003) at Konza Prairie

Long Term Ecological Research site in the Kansas Flint Hills.

Further, Heisler et al. (2003) found that early spring burning

at a 4-year interval permitted extensive establishment of new

shrubs from seed. We suggest that less intense, even if more

frequent, fires created conditions under which oak popula-

tions could expand at our study sites.

In concert with reduced fire intensities as a result of

drought and changes in fire season, large changes in land use

in the 1930s may have allowed oak population expansion.

Economic depression in the United States in the 1930s

forced many Kansas residents to sell land or forfeit land to

banks, leaving land unmanaged for prolonged periods

(Hornbeck, 2012). Further, positive attitudes towards trees as

a means to limit erosion during the 1930s ‘Dust Bowl’ may

have accelerated woody plant establishment. For example,

the Prairie States Forestry Project (1935–42) planted > 217

million trees and more than 28,000 km of shelterbelt

throughout the Great Plains (Droze, 1977). In this context,

oaks may have been allowed to expand to create additional

erosion control and protection for pastures.

‘Mesophication’ of Chautauqua Hills oak woodlands

Oak recruitment began to decline at our study sites between

1960 and 1990 and low recruitment rates persist. Canopy

closure and low light availability probably contributed to

recent, limited recruitment. In xeric oak woodlands of wes-


2112


tern North America, light limitation often results from self-

shading by adult oaks (Mensing, 1992). Oak recruitment

could resume after overstorey oaks die if juvenile oaks are in

the understorey. However, in eastern North America, light

limitation and failure of oak recruitment are associated with

the formation of dense understories of fire-intolerant, shade-

tolerant tree species that out-compete juvenile oaks for light

(Nowacki & Abrams, 2008). Nowacki & Abrams (2008)

hypothesized that oak woodlands may be a stable community

only on xeric sites in eastern North America. At the prairie–

deciduous forest ecotone in the Kansas Flint Hills, however,

even on steep, dry sites, declines in oak recruitment coin-

cided with increased recruitment of mesophytic tree species

(Abrams, 1986). Further, xeric oak woodlands in Oklahoma

do not appear to be stable as understories are dominated by

eastern red cedar, suggesting that when canopy oaks die they

will be replaced by eastern red cedars (DeSantis et al., 2011).

Our results suggest the hypothesis that slope aspect may

strongly influence the stability of oak woodlands in the

Chautauqua Hills. At our two sites with mesic, north- or

east-facing slopes, Cross Timbers SP and Fall River, meso-

phytic species, such as black cherry and fire-intolerant east-

ern red cedar, are well represented in the understorey.

Increases in mesophytic species in these woodlands are likely

to have occurred within the past 50 years. Hale (1955)

reported black cherry, hackberry (Celtis occidentalis) and

black walnut (Juglans nigra) only at very low densities in six

Chautauqua Hills oak woodlands that he sampled in 1954.

Oaks constituted > 97.5% of stems in all woodlands that

Hale (1955) sampled, whereas we found that 17% and 60%

of stems sampled were not oaks at Fall River and Cross Tim-

bers SP, respectively. Recent establishment of mesophytic tree

species at our study sites with mesic slopes is also indicated

by the much smaller diameters of these trees in comparison

with the oaks. By contrast, at our south- and west-facing

sites we encountered only oaks, suggesting greater stability at

these sites, consistent with Nowacki & Abrams’ (2008)

hypothesis. A note of caution, however, is required in assert-

ing the stability of oak woodlands on xeric slopes in the

Chautauqua Hills because fire is still used in management at

all of our study sites. These prescribed burns, however, only

burn into the edges of woodlands. For ecosystem restoration,

understanding where and when oak woodlands are stable is

important because savannas and open woodlands can be re-

established through thinning if oaks persist, but restoration

would be more challenging if a shift to a mesophytic alter-

nate stable state has occurred and oaks are lost entirely.

Landscape mosaics of grassland, savanna and woodland

are highly dynamic inherently. Understanding causes of shifts

in patch physiognomy within these mosaics and how human

land use influences such shifts has both economic and con-

servation implications. Our results support a growing body

of evidence for the counterintuitive conclusion that dry con-

ditions, even where tree species occur at their xeric range

limit, may allow woodland expansion. Further, our results

suggest the influence of aspect and identity of the woody

plant species in modifying effects of dry conditions on

woody plant population expansion. At this xeric range limit

for eastern oak species, aspect also appears to be important

in determining the stability of the oak woodlands that form.

Such results may enhance opportunities for spatially and

temporally targeted strategies to manage woody plant expan-

sion for economic and conservation goals.

ACKNOWLEDGEMENTS

We thank Randall Rogers for his consistent help with field-

work. We thank James Beck, Karen Brown, Greg Houseman,

Mary Liz Jameson, Oliver Keller, Maria Martino, Rob

McMinn, David Wickell and two anonymous referees for

insightful comments on early drafts of the manuscript. The

Stotts family, the Kansas Department of Parks, Wildlife and

Tourism, and the Army Corps of Engineers were of invaluable

assistance by providing access to study sites. We thank the

Kansas Historical Society for access to Kansas state censuses.

We are grateful to the Kansas Academy of Science Student

Research Grant Program and the High Plains Regional Climate

Center for funding.

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BIOSKETCHES

Thomas R. Rogers is interested in forest population and

community ecology. Recent research has focused on oak

population dynamics in woodlands and savannas, relation-

ships between landscape topography and tree species distri-

butions, and woody plant encroachment.

F. Leland Russell’s research focuses upon plant population

ecology and plant–animal interactions. Recent topics addressed

by Russell and colleagues include herbivore effects on plant pop-

ulation growth rates, spatial and temporal variation in herbivore

effects on host plants, tree population dynamics in woodlands

and savannas, and understorey restoration in oak savannas.

Editor: Miles Silman


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Documents

Historical patterns of oak population expansion in the Chautauqua Hills, Kansas